This is a rare phenomenon called iridescent clouds. It occurs in the clouds with ...
small droplets or ice crystals
The effect is similar to irisation. Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.
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+1 Very interesting! Kampfgraff
+1 Very interesting! Fabienne
+1 Very interesting! M